1. Field of the Invention
The present invention relates generally to radio frequency (RF) components. More particularly, the present invention relates to couplers that couple RF signals, including ultra high frequency signals, through a medium such as air, glass or other dielectric.
2. Background of the Invention
Through-glass couplers, as explained in, e.g., U.S. Pat. No. 5,565,877 to Du et al., are employed to RF couple two antenna modules that are mounted, respectively, on the outside and inside surfaces of window glass, such as automobile glass, to transmit signals through the window glass between the opposing modules. The outside antenna module might include a vertically extending antenna element, while the inside antenna module typically contains a connector or transmission feedline, which leads to a device such as a telephone, pager, facsimile machine, radio receiver, or the like, inside the vehicle. In a radio receiver implementation, the inside antenna module receives RF energy through the glass from the outside antenna module.
Loss occurs in glass mount antennas due to the dielectric material interposed between the inside and outside modules, as well as impedance mismatching. Therefore, a window glass mount antenna typically has lower gain compared to a non-through-glass antenna. However, conventional (i.e., non-through-glass coupled) antennas are less desirable because there must be a physical connection that extends through the body of a vehicle, between inside and outside antenna modules.
Conventional through-glass couplers employ capacitive coupling to transmit RF signals through the glass. In capacitively coupled antennas, two metal plates are positioned opposite each other on opposing surfaces of the window glass. These metal plates cooperate and act as a capacitor to transmit RF energy through the intervening window glass. However, to achieve better responses, especially at relatively higher frequencies, microstrip antennas have been adopted in certain applications, as exemplified by U.S. Pat. No. 5,565,877 to Du et al. There are many variations to microstrip antenna designs, as exemplified by, e.g., U.S. Pat. No. 4,130,822 to Conroy, U.S. Pat. No. 4,197,545 to Favaloro et al., U.S. Pat. No. 4,792,809 to Gilbert et al. and U.S. Pat. No. 5,793,263 to Pozar, but because of the wide array of applications for which microstrip antennas can be used, there is significant room for improvement in microstrip antenna design, particularly in specialized applications.
FIG. 1 illustrates a typical application for which a through-glass coupler is employed. In the case of, for example, a radio receiver implementation (although the same principles apply to a radio transceiver implementation) an antenna 10, receives a broadcast signal, which is applied to an outside module 200 of a through glass coupler 12. Outside module 200 is positioned against glass 14 and opposite inside module 100 on the opposite side of the glass 14. In some applications, a matching circuit 16 is preferably provided to match impedance values of the two complementary modules 100, 200. A radio frequency (RF) cable 18, e.g., coaxial cable, typically connects matching circuit 16 to a low noise amplifier (LNA) 20, which feeds receiver 22.
Of the known methods of transferring RF energy through glass, capacitive coupling, slot coupling, and aperture coupling represent the most common. However, an inherent drawback of all these coupling methods is that they increase the system noise due to relatively high RF coupling loss. To reduce coupling losses, the methods listed above need to be implemented on expensive circuit board ceramic material (i.e., Rogers 3003, 4003, 3010, etc.). The price of these materials, however, is significantly higher than that of, e.g., standard FR-4 printed circuit board. Thus, using low-loss type boards would make a consumer product very expensive.
Also, a typical slot coupler, as shown in FIG. 2, includes a circuit board 50, a microstrip feed line 52 and a slot 54 that exposes the underlying microstrip feed line 52. Such a device requires elaborate construction techniques, and may require the use of relatively expensive multi-layer boards. There is a need, therefore, for providing a less expensive coupler, yet one that provides the performance that matches or even exceeds known devices that are constructed using higher cost materials.
It is therefore an object of the present invention to provide a low cost yet capable through glass coupler.
It is another object of the present invention to provide a coupler that is simple to manufacture.
It is yet another object of the present invention to provide a through glass coupler that has inside and outside modules having asymmetrical configurations.
It is also an object of the present invention to provide components of a pair of through glass couplers on a single board.
It is also an object of the present invention to provide a through glass coupler that can be constructed using well-known etching techniques and low-cost copper clad circuit board material.
It is still another object of the present invention to provide a through glass coupler having a low profile design.
It is also an object of the present invention to provide a through glass coupler that not only has a low profile design, but also does not require a cavity, i.e., a slot.
To achieve the foregoing and other objects, an embodiment of the present invention comprises a pair of single layer double sided copper clad boards that are etched to include apertures and exciter strips that have different configurations. In a particular application for the through glass coupler of the invention, each copper clad board is etched to include components of two couplers, whereby two antennas or frequency bands can be accommodated and coupled.
Further in accordance with embodiments of the invention, the through glass coupler comprises a single layer design, thereby substantially facilitating the manufacture thereof. Additionally, no cavities are required, thereby achieving further savings in manufacturing costs and space.